1. Field of the Invention
The present invention relates to a field-emission electron gun, and electron-beam instruments mounting the same electron gun thereon.
2. Description of the Related Art
FIG. 1 illustrates a time-lapse change in the emission current in a conventional field-emission electron gun including a single-crystal tungsten cathode. If the electron gun continues to be used for 2 to 12 hours, as illustrated in FIG. 1, the emission current becomes unstable because of a configuration change in the cathode surface and gas adherence onto the cathode surface. This, eventually, results in a breakage of the tungsten cathode itself. This situation requires execution of a heat flashing processing (which, hereinafter, will be referred to as “flashing”) for current-pass heating the tungsten cathode periodically. The execution of the flashing melts away the cathode surface, thereby renovating the asperities which are formed by ion impact, and removing the gas molecules which adhere onto the cathode surface. This execution is considered to stabilize the emission current. A technology concerning this flashing has been disclosed in JP-A-11-144663.
FIG. 2 is a configuration diagram of the conventional field-emission electron gun. Here, the reference numerals denote the following configuration components: 1 a single-crystal tungsten cathode, 2 an electrically-conductive filament, 3 an electrode support base, 4 electrodes, 5 a heat flashing power-supply, 6 a heat flashing switch, 7 an extracting power-supply, 8 an accelerating power-supply, 9 an extractor, and 10 an accelerating electrode. As illustrated in FIG. 1, at the time of the flashing, it is required to stop the emission current once by dropping down extracting voltage and accelerating voltage. This makes it impossible to implement long-time continuous operation of the conventional field-emission electron gun. On account of this, despite the fact that the electron gun has supreme performance as a conventional electron source, its application range is limited to a high-resolution electron microscope for research purpose. As a consequence, there exists a serious problem that the electron gun is inapplicable to inspection devices such as a length-measuring SEM which is continuously driven on a semiconductor inspection line or the like.
Also, as illustrated in FIG. 1, the emission current decreases after the flashing, and it takes 0.5 to 3 hours to attain to a stable region of the emission current. As a result, maintaining the emission current at the constant value requires that the extracting voltage be boosted. This voltage-boosting changes ratio between the extracting voltage and initial-stage accelerating voltage, thereby changing electrostatic-lens function as well, and thus shifting optical axis. As a solving measure therefor, the following method has been disclosed in JP-A-6-162978: Namely, the extracting voltage is made variable so that the emission current is maintained at the constant value, and at the same time, the initial-stage accelerating voltage is changed in synchronization with the change in the extracting voltage. This processing allows the electrostatic-lens function to remain unchanged, thereby eliminating the shift of the optical axis of the electron-gun unit. It is impossible, however, to solve the problem of an optical-axis shift caused by heat deformation of the cathode itself due to repetition of the flashing. In this way, it is required to execute the flashing characteristic of the field-emission electron gun, and an optical-axis adjustment processing in accompaniment therewith. Accordingly, even if the electron gun is applied to a general-purpose SEM, the electron gun presents the problem in a point of its ease-of-use.